Conventionally, there has been known a MOSFET which includes a semiconductor base substrate having a super junction structure which is formed of an n-type column region and a p-type column region (see patent document 1, for example).
In this specification, “super junction structure” means a structure where an n-type column region and a p-type column region are alternately and repeatedly arranged as viewed in a predetermined cross section.
As shown in FIG. 17, a conventional MOSFET 900 is a planar-gate-type MOSFET which includes: a semiconductor base substrate 910 having a super junction structure 917 formed of an n-type column region 914 and a p-type column region 916, a base region 918 formed on a surface of a first main surface and a whole surface of the p-type column region 916 and a part of a surface of the n-type column region 914, an n-type surface high concentration region 919 formed on the surface of the first main surface and the surface of the n-type column region 914 such that the n-type surface high concentration region 919 is disposed adjacently to the base region 918, and an n-type source region 920 formed on a surface of the base region 918; and a gate electrode 936 formed on the surface of the base region 918 sandwiched between the source region 920 and the n-type surface high concentration region 919 by way of a gate insulation film 93.
In the conventional MOSFET 900, the n-type column region 914 and the p-type column region 916 are formed such that a total amount of a dopant in the n-type column region 914 is equal to a total amount of a dopant in the p-type column region 916. That is, the n-type column region 914 and the p-type column region 916 are well-balanced with each other in terms of a charge. Both dopant concentration in the n-type column region 914 and dopant concentration in the p-type column region 916 are set to a fixed value regardless of a depth of the n-type column region 914 and a depth of the p-type column region 916. Further, side walls of the n-type column region 914 are formed in a tapered shape narrowed toward a first main surface side, and side walls of the p-type column region 916 are formed in a tapered shape with a narrow bottom.
In the specification, “total amount of a dopant” means a total amount of a dopant in a constitutional element (the n-type column region or the p-type column region) in the MOSFET.
According to the conventional MOSFET 900, the MOSFET includes the semiconductor base substrate 910 having the super junction structure 917 which is formed of the n-type column region 914 and the p-type column region 916 and hence, it is possible to provide a switching element having a low ON resistance and a high withstand voltage.